1. Field of the Invention
This invention relates to lasers and, more particularly, to broadly turnable chromium-doped beryllium aluminate lasers and their operation.
2. Description of the Prior Art
Laser materials depend on their ability to absorb energy in such a way that more atoms (or molecules) are excited into a higher energy level than are in the terminal state, rendering the material capable of "stimulated" emission. If light of the appropriate wavelength is incident on such an excited material, it will stimulate emission of additional light having the same wavelength, phase, and direction as the incident light. This additional light, by augmenting the intensity of the incident light, demonstrates light amplification. Laser action was first demonstrated in 1960 using ruby (Al.sub.2 O.sub.3 :Cr.sup.3+), a crystalline solid system. Since then, laser action has been achieved in gas, liquid, and glass systems as well as in other crystalline solid systems.
All lasers are wavelength tunable, although most can be tuned only over a bandwidth of less than a nanometer. Broadly wavelength-tunable lasers (i.e., those with a broad tunable range) are extremely useful in scientific and industrial applications, e.g. isotope separation, but have been primarily restricted to the liquid medium dye lasers. There exists, however, a little known category of broadly tunable solid-state host-type lasers referred to as "phonon-terminated" or "vibronic" lasers.
Dye lasers, the most common broadly tunable lasers, have several shortcomings. They are generally limited in average power capabilities because of thermal instability of their liquid host, which also necessitates, for high power application, auxiliary equipment to maintain dye flow. Their lifetime is limited by decomposition of the organic dye material. Some of the dyes and solvents are toxic and/or corrosive. In addition, dye lasers essentially cannot be Q-switched.
Host-type lasers employ a dopant ion incorporated in dilute concentration in a solid host as the laser-active component. Vibronic lasers are a variety of host-type lasers which derive their tunability from emission of vibrational quanta (phonons) concurrent with the emission of light quanta (photons). The energies of the photons and phonons which are emitted simultaneously in a vibronic laser add up to the energy of the associated purely electronic or "no-phonon" transition. The broad wavelength tunability of a vibronic laser derives from the broad energy phonon continuum which complements the photon emission.
Several vibronic lasers have been described by L. F. Johnson, et al. in "Phonon-Terminated Optical Masers," Phys. Rev. 149, 179, 1966. All of these suffer from severe tuning discontinuities associated with structure in their vibronic spectra. In addition, they must be operated at low temperatures. Room temperature vibronic laser emission has been observed in the infrared at 2.17 .mu.m in Ho.sup.3+ -doped BaY.sub.2 F.sub.8 (L. F. Johnson and H. J. Guggenheim, "Electronic- and Phonon-Terminated Laser Emission from Ho.sup.3+ in BaY.sub.2 F.sub.8," IEEE J. Quantum Electron. QE-10, 442, 1974). The pulse threshold in this laser, however, is quite high--450 J. Room temperature vibronic laser emission in the vacuum ultraviolet has been predicted in Nd.sup.3+ -, Er.sup.3+ -, and Tm.sup.3+ - doped trifluorides (K. H. Yang and J. A. DeLuca, "VUV Fluorescence of Nd.sup.3+ -, Er.sup.3+ -, and Tm.sup.3+ -Doped Trifluorides and Tunable Coherent Sources from 1650 to 2600 A," Appl. Phys. Lett. 29, 499, 1976) The anticipated tunable laser emission from these materials, however, would require laser pumping because of the short fluorescence lifetime.
A broad-band room temperature solid state laser operating in the visible has been reported, although tunable laser action has only been predicted. (R. W. Boyd, J. F. Owen and K. J. Teegarden, "Laser Action of M Centers in Lithium Fluoride," IEEE J. Quantum Electron. QE-14, 697, 1978).
Tuning the output wavelength of a tunable laser can be accomplished by including within the optical resonator any optical element with wavelength selective properties, so that only light having a certain wavelength will be favored for amplification by the active medium. The only restriction on the selection of this element is that it not introduce at the selected wavelength optical loss of such magnitude as to prevent laser oscillation. One example of such an element is a prism placed within the optical cavity between the laser medium and the mirror at one end. The prism refracts different wavelengths of light at different angles. For laser oscillation to be maintained between the end mirrors forming the optical resonator, the mirrors must be perfectly aligned so that reflected light is perpendicular to their surface. Thus, by pivoting one of these optical resonator mirrors relative to the light passing through the prism, it is possible to restrict laser oscillation to a limited and selectable part of the emission spectrum.
Alternative methods for tuning employ in place of the prism an adjustable optical grating or a birefringent filter consisting of one or more birefringent plates which can be pivoted or rotated to select the output wavelength.
Tuning can also be accomplished by placing at the output end of the laser a lens having longitudinal chromatic aberration. Moving the lens toward and away from the laser medium changes the wavelength of laser emission.
The spectral width of the emitted radiation can be controlled over a wide range by the judicious selection of tuning elements to be included within the resonator. By employing elements with successively narrower transmission bandwidths, e.g. multiple etalons, the laser output can be narrowed to a single oscillating mode of the laser resonator.
Bukin, et al. (Sov. J. Quantum Electron. 8(5), 671, May 1978) reported on stimulated emission from alexandrite at 77 K. Output wavelength was variable over a range of less than 0.5 nm altering the crystal temperature.
U.S. Pat. No. 3,997,853, issued Dec. 14, 1976 to R. C. Morris and C. F. Cline, discloses laser emission at a wavelength of 6804 A (680.4 nm) from single crystals of trivalent chromium-doped beryllium aluminate oriented substantially along the a-c plane and having a Cr.sup.3+ doping concentration ranging from about 0.005 to 1.0 atom percent. These crystals have the chrysoberyl structure, an orthorhombic structure that is isomorphous with olivine. The space group of the structure is Pnma with lattice parameters a=0.9404 nm, b=0.5476 nm, and c=0.4427 nm.